The present invention relates to a driver circuit for a piezoelectric transformer, and more particularly to a driver circuit for a piezoelectric transformer to be used as a back-light power source of a liquid crystal display device.
The piezoelectric transformer is a voltage converter which shows a mechanical vibration due to piezo effect so that a converted or transformed voltage appears on a secondary side of the converter. A piezoelectric vibrator is made of a piezoelectric material showing piezo effect. Primary and secondary electrodes are provided at primary and secondary sides of the piezoelectric vibrator. The primary electrode is applied with a driving voltage which has the same frequency as a resonant frequency of the piezoelectric resonator so that the piezoelectric resonator exhibits a vibration at the resonant frequency. The piezoelectric transformer is superior in allowability of scaling down and thickness reduction as compared to an electromagnetic transformer, for which reason the piezoelectric transformer is suitable for a back-light power source in the liquid crystal display device.
A conventional driving circuit for such a piezoelectric transformer is disclosed in the Japanese laid-open patent publication No. 7-177736. The conventional driving circuit and the piezoelectric transformer have a protection feature which prevents them from substantial damage. FIG. 1 is a block diagram illustrative of the conventional driving circuit for the piezoelectric transformer. The driver circuit has a piezoelectric transformer 4 which has a primary electrode applied with an alternating current and a secondary electrode on which an output voltage appears which has been transformed by the piezoelectric effect. A power source VDD is provided which is electrically connected through a power switch 406 to the primary electrode of the piezoelectric transformer 4. An output voltage detector 403 is also provided which is electrically connected to the secondary electrode of the piezoelectric transformer 4. A comparator 404 is also provided which is electrically connected to the output voltage detector 403 for comparison of the detected output voltage to a reference voltage. A control unit 405 is further provided which is connected to the power switch 406 and the comparator 404 for sending the power switch 406 a control signal based upon the result of the comparison. A display device 407 is furthermore provided which is electrically connected to the comparator 404 for displaying the result of the comparison. A load 5 is also provided between the secondary electrode of the piezoelectric transformer 4 and the ground.
If the detected output voltage level is beyond the reference voltage, the control unit 405 sends the power switch 406 an instruction to stop the power supply.
FIG. 2 is a circuit diagram illustrative of the output voltage detector 403, the comparator 404 and the display device 407 of FIG. 1. Operations of the conventional driver circuit will be described as follows. A comparator 503 is provided in the comparator 404 for outputting a high voltage level when the output voltage level of the piezoelectric transformer 4 is below the available range. A comparator 505 is provided in the comparator 404 for outputting a high voltage level when the output voltage level of the piezoelectric transformer 4 is above the available range. A comparator 504 is provided in the comparator 404 for outputting a high voltage level when the output voltage level of the piezoelectric transformer 4 is in the available range.
If, for example, the piezoelectric transformer 4 is broken and the output voltage appearing on the secondary electrode is dropped to a lower level than the available voltage range, then the comparator 503 outputs the high level signal to send an OFF signal through a latch 506 to the power switch 406. If the load 5 is opened and the load impedance rapidly increases, then the output voltage level of the piezoelectric transformer 4 is rapidly increased whereby the piezoelectric transformer 4 exhibits an excess vibration. As a result, the output voltage level from the secondary electrode of the piezoelectric transformer 4 is beyond the available voltage range. In this case, the comparator 505 outputs a high level signal which is transmitted through the latch 507 to a light emitting diode 510 whereby the light emitting diode 510 enters into the light-ON as well as send OFF signal to the power switch 406. If the output voltage is in the available range, then the comparator 504 is operated and the light emitting diode 509 lights ON. If the piezoelectric transformer 4 is broken, then the protection circuit is operated to stop the power supply to prevent a discharge from the broken part thereof.
The above protection circuit described above has the following problems. The first problem is that even if no break appears but an output voltage is dropped to a lower voltage level than the available voltage range, the power switch turns OFF and will remain in the OFF state. The above piezoelectric transformer is the voltage converter which shows a mechanical vibration due to piezo effect so that a converted or transformed voltage appears on a secondary side of the converter. Immediately after the power-on, it takes a time to achieve the predetermined output voltage level.
FIG. 3 is a time chart showing one characteristic of rising up of the driving of the piezoelectric transformer 4. The necessary driving time duration, or a time duration, from the initiation of the supply of a driving voltage with a resonant frequency to a predetermined output voltage level was about 150 microseconds. The driving circuit for the piezoelectric transformer sweeps the driving frequency of the piezoelectric transformer to set the frequency at an appropriate driving frequency. Actually, this sweeping time is essential. The sweeping time depends upon the driving circuit and is, for example, a few milliseconds to several tens of milliseconds. For this reason, immediately after the power-on, no output voltage is generated. At this time, the conventional driving circuit is considered to be in the abnormal state and then the power switch turns OFF and will remain the OFF state.
If a battery is used as the power source, there is a problem in any insufficient power supply due to consumption of the power. In this case, if the driving is made at the resonant frequency allowing a maximum power voltage, it might be difficult to generate the necessary voltage level for lighting a cathode tube. In this case, the power switch turns OFF to discontinue the power supply. Even if the power source voltage is returned to a rated value, then the power switch remains OFF whereby the cathode tube does not light ON.
If the output voltage is above the available range, then the power switch 406 turns OFF and will remain in the OFF state. If, for example, no light-ON state can be obtained due to a high impedance of the cathode tube on the ground of a low atmospheric temperature. In case of the load opening state due to electrical content imperfection, the output voltage level of the piezoelectric transformer is rapidly increased up to a higher voltage level than the available voltage range whereby the power supply is discontinued. Even if the atmospheric temperature is increased or the electrical contact perfection can be obtained, then the power switch remains OFF whereby the cathode tube will not turn ON.
In the above circumstances, it had been required to develop a novel driving circuit for a piezoelectric transformer free from the above problems.